Sonic Fluid

How Fluid Machines Work
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We spoke a lot about subsonic and supersonic fluids in a converging and diverging vessel. At what point is the fluid sonic?

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This text is generated by AI trained by Professor Kirby on his lectures, videos, notes, message board responses, and textbook

PK Bot’s answer:
In a converging-diverging nozzle in which the flow is choked, the fluid becomes sonic at the throat, which is the narrowest part of the nozzle. This is the point where the Mach number M equals 1.

For subsonic flow entering the nozzle, as the fluid accelerates through the converging section, the velocity increases and the Mach number approaches 1. At the throat, the flow reaches the speed of sound, and the Mach number is exactly 1, making the flow sonic. Beyond the throat, in the diverging section, the flow can become supersonic if the pressure conditions are appropriate, allowing the Mach number to exceed 1.

This transition from subsonic to sonic flow at the throat is a critical aspect of nozzle design, particularly in applications like rocket engines, where achieving supersonic flow is needed for maximum thrust.

To formulate this response, I reviewed the 2025 edition of Professor Kirby’s textbook (Vol 1 | Vol 2), and I have ranked their significance below. If they are marked as relevant, I encourage you to explore them:

Textbook sections:

  1. Chapter 1.4: Properties of sound waves (MARGINALLY RELEVANT)
  2. Chapter 1.3: Acoustics and Compressible Flow (MARGINALLY RELEVANT)
  3. Chapter 1.2: 1D Conservation Equations (MARGINALLY RELEVANT)
  4. Chapter 9.4: Finding Analytical Solutions for \psi and \phi Part 4 (MARGINALLY RELEVANT)
  5. Chapter 1.5: Isentropic quasi-1D flow relations Part 1 (MARGINALLY RELEVANT)

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